If you found this post, you are probably trying to figure out why your Volvo XC90 radio is not working. Sometimes, the problem is water leaking from the sunroof drain tubes and affecting the amplifier under the front passenger seat. Others have suggested that the head unit itself goes bad. Keep searching and you may find that it is something altogether different. (Someone mentioned the door module.)
Most of us are not interested in replacing parts until the radio starts working again. That could get expensive quickly. However, if you have some experience with installing radios, you might follow the instructions provided in this video: